Field of the Invention
This invention relates to a material for nonmagnetic oxide substrate to be used in a magnetic head coated with a soft magnetic thin film and to a magnetic head using the material.
Description of the Prior Art
As a magnetic head to be incorporated in video tape recorders ("VTR") and digital audio tape ("DAT") devices, for example, a thin-film magnetic head having a magnetic thin film made, for example, of Sendust or amorphous magnetic alloy formed on the surface of a substratal material, has found extensive utility.
In the thin-film magnetic head, the substratal material thereof is required to have a thermal expansion coefficient which closely approximates the thermal expansion coefficient of the magnetic thin film. If a large difference exists between the thermal expansion coefficients of the two components, a change in temperature has the possibility of imparting stress to the interface or union of the two materials, inducing a crack therein, and impairing the magnetic properties of the magnetic head. As necessary properties, the substratal material must maintain smoothness of surface and posses proper abrasiveness relative to the magnetic tape and exhibit ideal sliding and abrading properties to the magnetic tape.
The feasibility of cobalt (Co) type amorphous thin films and Sendust thin films as magnetic thin films of outstanding magnetic properties has been the subject of considerable study. The Co type amorphous alloys have a thermal expansion coefficient in the range of from 100 to 120.times.10.sup.-7 /.degree.C. and the Sendust has a thermal expansion coefficient of not less than 120.times.10.sup.-7 /.degree.C. The substratal material, therefore, is required to possess a thermal expansion coefficient closely approximating that of the thin film being considered.
As substratal materials which fulfil this requirement, NiO-TiO.sub.2 type oxide materials [Japanese Patent Application Disclosure (KOKAI) SHO 62(1987)-95,810, SHO 60(1985)-204,668, SHO 60(1985)-204,699, SHO 60(1985)-246,258, SHO 60(1985)-246,259, SHO 60(1985)-264,362, SHO 60(1985)-264,363, and SHO 62(1987)-143,857] and NiO-CoO-TiO.sub.2 type oxide materials [Japanese Patent Application Disclosure (KOKAI) HEI 2(1990)-154,307] have been known to the art.
Japanese Patent Application Disclosure (KOKAI) SHO 62(1987)-95,810 discloses a substratal material which is composed of 50 to 90 wt % of NiO and the balance of TiO.sub.2 and a substratal material which incorporates a small amount of ZrO.sub.2 in addition to NiO and TiO.sub.2. The other disclosures teach substratal materials of the NiO-TiO.sub.2 type additionally incorporating therein CaO, MgO, Al.sub.2 O.sub.3, ZrO.sub.2, Cr.sub.2 O.sub.3, CaMnO.sub.3, etc.
Japanese Patent Application Disclosure (KOKAI) HEI 2(1990)-154,307 discloses a substratal material which is composed of 25 to 85 mol % of Co, 0 to 50 mol % of NiO, and 5 to 20 mol % of TiO.sub.2.
These known substratal materials are produced with thermal expansion coefficients closely approximating those of magnetic thin film alloys. It has been recently pointed out, however, they are deficient in smoothness of surface and abrasiveness relative to the magnetic tape and are susceptible to loss of smoothness and uneven wear against the magnetic thin film.
A magnetic head using a substratal material of a nonmagnetic substance and incorporating therein a magnetic thin film as an equivalent of a gap has been disclosed by Japanese Patent Application Disclosure (KOKAI) SHO 60(1985)-231,903, for example. This magnetic head is so constructed that at least two opposite lateral surfaces and an operating gap-forming surface of a projected part having a cross section protruding substantially in a V shape toward the magnetic recording medium are coated with a metallic magnetic substance and the metallic magnetic coating is opposed at the leading end of the projected part to the magnetic recording medium through the medium of the operating gap.
As a nonmagnetic substratal material matching a thermal expansion coefficient with the Co type amorphous thin film, the present inventors formerly invented a material substantially comprising a NiTiO.sub.3 phase and NiO phase and having a composition of 60 to 77 wt % of NiO and 40 to 23 wt % of TiO.sub.2 [Japanese Patent Application Disclosure (KOKAI) HEI 2(1990)-81,503]. Even the material of this composition, however, has the disadvantage in that it suffers a serious loss of surface smoothness because of its contact with the magnetic tape and it wants further improvement in abrasiveness.
An object of this invention is to provide a substratal material for the magnetic head which solves the problem of loss of surface smoothness during the slide with the magnetic tape and matches its wear with the magnetic thin film during the motion of the magnetic tape and consequently provide a magnetic head which matches its sliding and abrading property with the magnetic tape.